Have the same problem on a Sony Vaio Laptop, Sandy Bridge i5-2450M with AMD switchable graphics.
I've tried everything I've found here and there, such as:
- Start (and run it) without the battery
- Remove the battery power manager in the devices manager
- Searched for the famous registry keys (which are not there in W10)
- Upgrade install (first), then fresh install (now)
- Enable Hyper-V
Basically, nothing is working, and my PC is locked at 0.78GHz, making it totally useless.
I was actually ready to give up on my AMD graphics (which I knew was never going to work in W10), but ... having the CPU locked to less than 1GHz is a no go ...
Since no-one seems to have solved this issue, I guess that I'll need to go back to W8.1 and forget about W10 for a while... Pretty sad indeed, but may well be one of the reasons why Sony had to give up on their PC business. They did build and design great
products, but always offered the worst possible service, trying as hard as they could to lock you down into buying new ones.